Snap-acting bimetal device



y 13, 58 s. R. HOOD 2,834,853

' SNAP-ACTING BIMETAL DEVIICE Filed April 12, 1957 "IT 5-; l

UnitedStates Patent Q SNAP-ACTING BIMETAL DEVICE Stanley R. Hood,Birmingham, Mich., assignor to Essex Wire Corporation Application April12, 1957, Serial No. 652,500

7 Claims. (Cl. 200-138) This invention relates to snap-acting devicesand more particularly to snap-acting bimetal elements useful inthermostatic switches.

An object of the invention is to provide a thermostatic device whichoperates with a positive snap action in both make and break action.

Another object of this invention is to provide a snapaeting thermostaticswitch having a relatively wide gap between the fixed and movablecontacts in the open position.

Known forms of thermostatic switches having the characteristicsmentioned above have employed a fiat bimetal element mounted at one endin a cantilever fashion with its opposite end which carries a contactmoving with a snap action between two stable positions. Generally such abimetal element is provided with three substantially parallel legs ofwhich the outer two are deformed to arch or bow the center leg in such amanner that the internal stresses developed by the bimetal withtemperature change cause the free end of the element to move over acomparatively large distance with a snap action.

These three-legged type of bimetal elements are not adapted forconvenient calibration or adjustment of their operating characteristicssuch as the make and break temperatures. Very small variations in thedeformation of the legs result in substantially large variations in thebimetal element operating characteristics and as a result, such bimetalelements have been diflicult to calibrate with operating characteristicswithin narrow limits or even with operating characteristics uniform fromone elemen to the next.

Accordingly a further object of the present invention is to provide anew form of snap-acting elements for thermostatic switches which isefficient in operation and which can be economically manufactured withuniformity in calibration.

Still another object is to provide a snap-acting thermostatic switch inwhich the stressing means are self-contained and which require noexternal springs or applied forces to produce a snap action.

Other objects and advantages of the invention will be apparent from thefollowing description and the accompanying drawing, in which:

Figure 1 is a plan view of a longitudinally curved bimetal blank.

Figure 2 is a longitudinal side view of the bimetal blank.

Figure 3 is a plan view of a snap-acting member formed from the bimetalblank of Figures 1 and 2.

Figure 4 is a longitudinal side view of the snap-acting member.

Figure 5 is a plan view of the snap-acting member secured to a supportin a switch assembly.

Figure 6 is a side elevation of Figure 5.

Figure 7 is a plan view of another form of the snapacting member.

Figure 8 is a longitudinal side view of the snap-acting member in Figure7.

Referring to Figures 1 and 2, there is shown a longitudinally curvedbimetal blank 10 having substantially triangular shape with a centralaperture or slot 11 which separates the blank 10 into a first stripportion 12 and a. second strip portion 13 integrally joined at theirseparated ends by a base portion 14. For mounting the snapacting memberto be formed from the blank in a switch assembly, the blank 10 isprovided with a hole 15 located in the base portion 14.

The blank 10 is a one piece stamping from a flat sheet of thermostaticbimetal material composed of two thin layers of metal bonded togetherand having different temperature coeflicients of expansion. For use inthe switch assembly of Figures 5 and 6, the high expansion layer ispreferably disposed on the lower or contactcarrying side of the blank10. The bimetal blank 10 is provided with a permanent longitudinallycurved or bowed form so that the high expansive side of the blank isconcave. The radius of curvature will vary with the snap and resettemperature requirements as well as the thickness, hardness, and type ofthermostatic bimetal employed.

Snap action in the bimetal member 16 of Figures 3 and 4 is accomplishedby stamping a permanently set small indentation or crimp 17 in thenarrow strip portion 13. This indentation draws the ends of the stripportion 13 closer together to produce a tension force in the narrowstrip portion 13 and a compression force in the wide strip portion 12.These forces together with that resulting from the longitudinalcurvature of the blank 10 stress the bimetal member 16 such that itssurface has a non-developable concave-convex form. This non-developablesurface form gives the snap-acting member a rigidity which in a switchassembly provides good contact pressure even under conditions of severevibration.

With strip portions having the correct dimensional proportions, forming,and crimping, the member 16 will move with a snap action when heated toa predetermined temperature. With the application of heat, the greaterexpansion of the metal on the concave surface of the bimetal member 16causes it to become convex at a predetermined temperature with apronounced snap action. When the bimetal cools sutficiently, the member16 snaps back to its original contour. The stressing means are entirelyself-contained and no external springs or applied forces are required toproduce snap-action.

It is important that the two strip portions 12 and 13 have the correctproportions. Strip portion 12 preferably should be 1.4 to 2.4 times aswide as strip portion 13. No snap action is obtained with bimetalmembers having width ratios much beyond these limits. It is alsopreferable that the narrow strip portion be longer than the wider stripportion in order that its length may change at a greater rate than thatof the wider strip portion.

Figures 7 and 8 illustrate a modification of the above bimetal member inwhich the triangular shape is more pronounced. The member 18 isconstructed in the same manner as member 16 shown in Figures 3 and 4,and accordingly, its parts have been designated with the same referencenumerals. Member 18 differs from member 16 only in its shape which issomewhat less suitable for extremely compact switch assemblies.

Figures 5 and 6 illustrate the use of the member 16 of Figures 3 and 4as part of a switch assembly such as nected to terminal 25. Sincethesurface of the member 16 id eiirved, the area of contact between thebimetal member 16 and contact 23 is kept to a minimum to avoid anyeffect on the operating characteristics of the bimetal em -.7

amount of longitudinal curvature of the blank 10 and the amount ofcrimping of the narrow strip portion 13 may be readily varied to controlthe snap or operating temperature and the reset temperature of thesnapacting member 16. The final curvature of the member 16 determinesthe snap temperature and is dependent upon both the longitudinalcurvature of the blank 10 and the crimping of the narrow strip portion13. Actually, the snap temperature is relatively independent of thelongitudinal curvature of the blank it) and varies mainly with changesin the depth ofjcrimping in the narrow strip portion113. Greater depthofcrimping in the narrow strip portion 13 results in a higher snaptemperature.

The reset temperature varies substantially with changes in either thelongitudinal curvature or the depth of crimping. For any particularamount of crimping, the reset temperature will decrease with a decreasein longitudinal curvature. If the longitudinal curvature of the blank 10is fixed, the reset temperature will decrease with increase in the depthof the crimp 17 in the narrow strip portion 13. I i i 7 By properlycorrelating the longitudinal curvature of the blank 10 with the depth ofcrimping in the narrow strip portion 13, both the snap andreset-temperatures may be readily maintained within narrow limits.Variations in the bimetal thickness, hardness, or characteristics areeasily compensated for by suitably adjusting the forming dies for thelongitudinal curvature and/or the crimp. Furthermore, no hand adjustmentor calibration of the switch assembly employing these snap-actingmembers is required as the blades are pro-formed with the desiredoperating characteristics.

While preferred embodiments of the invention have been illustrated, itshould be understood that other modifications may be made withoutdeparting from the spirit and the scope of the invention as defined inthe claims which follows.

I claim as my invention:

1. In a thermally responsive snap action device, a substantiallytriangular bimetal member having a base portion and cqnverging sidesmeeting at an apex portion, said bimetal member having an aperturecxtending longitudinally between said base and apex portions with edgessubstantially parallel to said converging sides to form a first stripportionon one side of said aperture and a second narrower strip portion(if greater length on the other side of said aper-ture, saidstripportions being longitudinally curved convexly from a common plane,and said second strip portion having atransversely extending crimpstressing said bimetal member into' a non-developable form.

2. Ir1 thermally responsive device, a slotted integral bimetal memberhaving converging first and second strip portions joined b y a baseportionat their separated ends, said second strip portion having agenerally uniform Width less than that of said first strip portion andhaving a length greater than that of said first strip portion, saidfirst and second strip portion being deformed to stress s aid bimetalinto a concavo-convex non-developable form, one of said strip portionsbeing in tension and the other being in compression to provide for snapaction of said bimetal member. I v

I 3. The bimetal member defined claim 2 and further eharaeterized bysaid first leg being 1.4 to 2.4 times as wide as sa d sesame leg.

4, thermally responsive switch comprising: a supportmernber; a firstcontact carried by said support member; a slotted integral bimetalmember of substantially triangular shape having a base portion andconverging Sides meeting at an apex portion, said bimetal member havingan aperture extending longitudinally between said base and apex safetiesedges substantially parallel to said converging sides to form a firststrip portion on one side of said aperture and a second narrower stripportion of greater length on the other side of said aperture, said stripportions being longitudinally curved convexly from a common plane, saidsecond strip portion having a transversely extending crimp stressingsaid bimetal member into a non developable form, said bimetal memberbeing supported at oneend thereof by said sup port member such that oneend of said base portion is freely flexible in the presence of thermalstresses for movement relative to the other end of said base portion;and a second contact actuated by said bimetal member co-operating withsaid first contact.

5. in a thermally responsive snap-action device, an elongated bimetalmember having a single longitudinally extendingaperture of substantiallytriangular shape separating said member into a first and a secondnon-parallel strip portions integrally joined together at their ends andhaving generally uniform widths, said first strip portion being widerand shorter than said second strip portion, said strip portions beinglongitudinally curved convcxly from a common plane, and said secondstrip portion being further deformed to stress said member into aconcaveconvex non-developable form.

6. In a thermally responsive snap action device, a sub stantiallytriangular bimetal member having a base portion and converging sidesmeeting at an apex portion, sa id bimetal member having an apertureextending longitudinally between saidbase and apex portions with edgessubstantially parallel to said converging sides to form a first stripportion on one side of said aperture and a second narrower strip portionof greater length on the other side of said aperture, said stripportions being initially longitudinally curved convexly from a commonplane, said second strip portion being additionally provided with atransversely extending crimp stressing said bimetal member into anon-develop able form, and the base portion adjacent one strip portionbeing freely flexible in the presence of thermal stresses for movementrelative to the base portion adjacent the other of said strip portions7. A thermally responsive switch comprising: a support member; a fixedcontact'carried by said support member; a bimetal member ofsubstantially triangular shape mounted upon said support member; amovable contact member carried by said bimetal member for engagementwith said fixed contact; said bimetal member comprising first and secondstrip portions joined by a base portion at their separated ends; saidsecond strip portion having a generally uniform width less than that ofsaid first strip portion and having a length greater than that of saidfirst strip portion; said first and second strip portions beinginitially formed with a longitudinally convex curvature away from acommon plane; said second strip portion being additionally formed with atransversely extending crimp buckling said bimetal member in a directionat anangle to said longitudinal convex curvature to stress said bimetalmember into a non-developable form; and means attaching said bimetalmember to said support member such that the base portion adjacent onestrip portion is freely flexible in the presence of thermal stresses formovement relative to the base portion adjacent the other of said stripportions.

References Cited in the file of this patent UNITED STATES PATENTS2,171,895 Sardeson Sept. 5, 1939 2,363,280 Arnold Nov. 21, 1944 2,425,717 Bean Aug 19', 1 947 2,432,488 Peacock et al Dec. 9, 1947

